Fix usr.bin.truncate.truncate_test.bad_truncate with ZFS /tmp.

The bad_truncate test sets the uimmutable flag to produce an error in
truncate, but that flag isn't supported by ZFS. If /tmp is on a ZFS
filesystem, the test will fail. Change it to use readonly permissions and
an unpriveleged user instead.

Reviewed by: jilles
MFC after: 1 week
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4862